1. Field of the Invention
The present invention relates to an image display apparatus, and in particular, an image display apparatus in which a plurality of pixels arranged in rows and columns in an image display portion are driven to display an image.
2. Description of the Background Art
Liquid crystal display apparatuses have been used widely in portable equipment such as cellular phones, as display devices with low power consumption. A liquid crystal display apparatus generally includes an image display portion having a plurality of pixels arranged in rows and columns, a horizontal scanning circuit applying a display voltage corresponding to display data to a plurality of source lines provided in the column direction corresponding to the pixels, and a vertical scanning circuit activating a plurality of gate lines provided in the row direction corresponding to the pixels. Gate lines are sequentially activated by the vertical scanning circuit, and the display voltage corresponding to the display data is supplied to the pixels connected to a row to be scanned, by the horizontal scanning circuit via the source lines. Consequently, a liquid crystal cell included in each pixel emits light with a display luminance corresponding to the supplied display voltage, and a desired image is displayed all over the image display portion.
In the portable equipment, a partial display function is known, in which an image is displayed only in a partial region of the image display portion and it is not displayed in the remaining region in a standby mode, to achieve even lower power consumption. With regards to this partial display function, a specific color (for example white or black) is generally displayed in a non-display region. There has been a problem that the horizontal and vertical scanning circuits operate also in the non-display region to display the specific color as in the display region, thus failing to reduce power consumption sufficiently.
In relation to such a problem, Japanese Patent Laying-Open No. 2001-343928 discloses an image display circuit for an image display apparatus equipped with a partial display function. The image display circuit includes an output control block controlling an output of an ON signal to each scanning signal line (equivalent to a gate line), such that scanning signals for display are output at one time to a plurality of scanning signal lines corresponding to a non-display region in response to a gate control signal for making a transition as for the output of the ON signal to each scanning signal line from sequential output to simultaneous output.
According to the image display apparatus, since a specific color is displayed in the non-display region simultaneously in the partial display function, a period during which an operation of a scanning signal line driving section is suppressed can be achieved after the simultaneous display, reducing power consumption of the scanning signal line driving section during the period.
Further, the so-called self-refresh function is known in the portable equipment to achieve lower power consumption as in the partial display function. By the self-refresh function, display data (a display voltage) is temporarily saved in each pixel and the saved data is used to rewrite the display data in a refresh operation, without supplying the display voltage corresponding to the display data from a horizontal scanning circuit.
It is possible for the self-refresh function to perform data rewrite simultaneously for all pixels in an image display portion, however, such a data rewrite simultaneously performed for all the pixels requires a driver large enough to drive all the pixels. Furthermore, the interconnection should be thicker to prevent malfunction due to noise caused by the simultaneous driving, resulting in an increase in the size of the apparatus.
For the above problem, a partial self-refresh function is known, which partially performs the self-refresh function for each block of an image display portion divided into blocks. In the partial self-refresh function, the image display portion is divided into blocks for a plurality of gate lines, for example. According to the partial self-refresh function, the number of pixels to be rewritten simultaneously is limited by the size of the block, causing no problems concerning the size of a driver and the size of an interconnection in the case where the self-refresh operation is simultaneously performed for all the pixels.
In both of the partial display function disclosed in Japanese Patent Laying-Open No. 2001-343928 and the conventional partial self-refresh function described above, it is necessary to simultaneously control some of a plurality of pixel control lines. That is, in the partial display function disclosed in Japanese Patent Laying-Open No. 2001-343928, a plurality of gate lines corresponding to the non-display region must be activated simultaneously, and in the conventional partial self-refresh function described above, a plurality of gate lines corresponding to a block to be refreshed must be activated simultaneously.
In the image display apparatus disclosed in Japanese Patent Laying-Open No. 2001-343928 described above, however, the output control block is additionally provided to implement the partial display function, causing an increase in the area of the apparatus.
In addition, in the conventional partial self-refresh function described above, a plurality of control signal lines and a plurality of buffer circuits corresponding to the control signal lines are additionally required to implement the function, rendering the control circuit complicated.